1. Field of the Invention
The invention relates generally to robotic mechanisms and, more particularly, relates to robotic mechanisms having two legs and methods of actuating.
2. Background Information
Conventional electric-powered biped robots such as humanoid robots maintain balance by having large feet and actively controlling their body posture. By adjusting the posture, they position their center directly above the foot touching the ground to achieve stability. This is made easier with large feet that provide large contact areas with the ground. In most embodiments, such robots rely on inertia sensing to actively control the location of the center of gravity and maintain it above the feet.
Such balancing control strategies provide static stability, i.e. the robot maintains its balance throughout its walking gait. Its movement can be interrupted at any time without loss of stability. One example is the zero moment point (ZMP) strategy, commonly used in humanoids.
Conventional biped robots have multiple electric motors installed in their legs, such as at the hip, knee and ankle. Consecutive segments of the leg are typically connected by motor-gear articulations, which enables motions of these segments one relative to the other. The coordinated actuation of these articulations makes the leg describe specific trajectories and generate locomotion gaits.
The limitations of conventional electric-powered biped design are two folds. First, actively adjusting the posture to achieve static stability leads to relatively slow motions that do not mimic biological gaits, and requires elaborate sensors and computation. Second, motor-gear articulations dissipate considerable energy every time the foot touches the ground, because these transmissions do not store and restore energy efficiently. When the foot impacts the ground, energy is lost to inelastic collisions, making it difficult for biped robots to run dynamically.
The present invention addresses both limitations by providing sensor-free static stability without large feet, and enabling dynamic running with elastic legs. The robot's body configuration is designed such that its center of gravity is permanently below the hips, which leads to static stability even in the absence of feet. The legs are actuated by a single motor at the hip, making the legs describe complete circles around the hips. The legs are shaped in a spiral to enable the robot to stand up by simple actuation of the hip motors. No sensing is needed for static stability or standing up.
Dynamic running is made possible by forming the legs out of compliant material. This provides the legs with elastic properties so they can efficiently store and restore energy each time the leg touches the ground. This efficient exchange of energy enables the biped robot to run dynamically.